1.. SPDX-License-Identifier: GPL-2.0-only 2 3GPIO Aggregator 4=============== 5 6The GPIO Aggregator provides a mechanism to aggregate GPIOs, and expose them as 7a new gpio_chip. This supports the following use cases. 8 9 10Aggregating GPIOs using Sysfs 11----------------------------- 12 13GPIO controllers are exported to userspace using /dev/gpiochip* character 14devices. Access control to these devices is provided by standard UNIX file 15system permissions, on an all-or-nothing basis: either a GPIO controller is 16accessible for a user, or it is not. 17 18The GPIO Aggregator provides access control for a set of one or more GPIOs, by 19aggregating them into a new gpio_chip, which can be assigned to a group or user 20using standard UNIX file ownership and permissions. Furthermore, this 21simplifies and hardens exporting GPIOs to a virtual machine, as the VM can just 22grab the full GPIO controller, and no longer needs to care about which GPIOs to 23grab and which not, reducing the attack surface. 24 25Aggregated GPIO controllers are instantiated and destroyed by writing to 26write-only attribute files in sysfs. 27 28 /sys/bus/platform/drivers/gpio-aggregator/ 29 30 "new_device" ... 31 Userspace may ask the kernel to instantiate an aggregated GPIO 32 controller by writing a string describing the GPIOs to 33 aggregate to the "new_device" file, using the format 34 35 .. code-block:: none 36 37 [<gpioA>] [<gpiochipB> <offsets>] ... 38 39 Where: 40 41 "<gpioA>" ... 42 is a GPIO line name, 43 44 "<gpiochipB>" ... 45 is a GPIO chip label, and 46 47 "<offsets>" ... 48 is a comma-separated list of GPIO offsets and/or 49 GPIO offset ranges denoted by dashes. 50 51 Example: Instantiate a new GPIO aggregator by aggregating GPIO 52 line 19 of "e6052000.gpio" and GPIO lines 20-21 of 53 "e6050000.gpio" into a new gpio_chip: 54 55 .. code-block:: sh 56 57 $ echo 'e6052000.gpio 19 e6050000.gpio 20-21' > new_device 58 59 "delete_device" ... 60 Userspace may ask the kernel to destroy an aggregated GPIO 61 controller after use by writing its device name to the 62 "delete_device" file. 63 64 Example: Destroy the previously-created aggregated GPIO 65 controller, assumed to be "gpio-aggregator.0": 66 67 .. code-block:: sh 68 69 $ echo gpio-aggregator.0 > delete_device 70 71 72Generic GPIO Driver 73------------------- 74 75The GPIO Aggregator can also be used as a generic driver for a simple 76GPIO-operated device described in DT, without a dedicated in-kernel driver. 77This is useful in industrial control, and is not unlike e.g. spidev, which 78allows the user to communicate with an SPI device from userspace. 79 80Binding a device to the GPIO Aggregator is performed either by modifying the 81gpio-aggregator driver, or by writing to the "driver_override" file in Sysfs. 82 83Example: If "door" is a GPIO-operated device described in DT, using its own 84compatible value:: 85 86 door { 87 compatible = "myvendor,mydoor"; 88 89 gpios = <&gpio2 19 GPIO_ACTIVE_HIGH>, 90 <&gpio2 20 GPIO_ACTIVE_LOW>; 91 gpio-line-names = "open", "lock"; 92 }; 93 94it can be bound to the GPIO Aggregator by either: 95 961. Adding its compatible value to ``gpio_aggregator_dt_ids[]``, 972. Binding manually using "driver_override": 98 99.. code-block:: sh 100 101 $ echo gpio-aggregator > /sys/bus/platform/devices/door/driver_override 102 $ echo door > /sys/bus/platform/drivers/gpio-aggregator/bind 103 104After that, a new gpiochip "door" has been created: 105 106.. code-block:: sh 107 108 $ gpioinfo door 109 gpiochip12 - 2 lines: 110 line 0: "open" unused input active-high 111 line 1: "lock" unused input active-high 112